Surface ozone exceedances in Melbourne, Australia are shown to be under NO_x control, as demonstrated using...

Two and a half years of multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of nitrogen dioxide (NO₂), formaldehyde (HCHO) and glyoxal (CHOCHO) are presented alongside in-situ ozone (O₃) measurements in Melbourne, Australia. Seasonal and diurnal cycles, vertical profiles and relationships with key meteorological variables are provided. NO₂ and CHOCHO were found at highest concentration for low wind speeds implying that their sources were predominantly localised and anthropogenic. HCHO showed an exponential relationship with temperature and a strong wind direction dependence from the northern and eastern sectors, and therefore most likely originated from oxidation of biogenic volatile organic compounds (VOCs) from surrounding forested and rural areas. The glyoxal:formaldehyde ratio (R_gf), reported for the first time in Australia, was consistently high compared to values elsewhere in the world with a mean of 0.105 ± 0.0503 and tended to increase with increasing anthropogenic influence. The HCHO:NO₂ ratio (R_fn) was used to characterise tropospheric ozone formation conditions. A strong relationship was found between high temperature, low R_gf, high R_fn and high ozone surface concentrations. Therefore, we propose that both R_gf and R_fn may be useful indicators of tropospheric ozone production regimes and concentrations. The R_fn showed that the vast majority of high ozone production episodes occurred under NO_x-limited conditions, suggesting that surface ozone pollution events in Melbourne could be curtailed using NO_x emission controls.